Aluminum anodizing



United States Patent No Drawing. Filed Nov. 28, 1961, Ser. No. 155,466

Claims. (31. 20458) This invention relates to anodizing aluminum, and particularly to producing oxide coatings on an aluminum article by making the same anode in an electrolytic cell employing an aqueous acid electrolyte. As used herein, the word aluminum includes high purity aluminum, aluminum in various commercial grades, and aluminum base alloys.

Decorative and protective oxide coatings on aluminum have long been made by anodizing in electrolytes consisting of aqueous solutions of sulfuric acid. Such electrolytes are usually employed at a temperature near room temperature, and must be cooled to maintain such temperature. The coatings, as so produced, are usually relatively clear, white or colorless, although on some alloys they may be tinted or colored by some constituents.

In recent years, hard anodic oxide coatings which are more abrasion resistant, have been produced by anodizing in aqueous sulfuric acid electrolytes at extremely low temperatures, e.g. 25 to 32 F., requiring much more cooling, with consequent increased equipment and operating expense. Such coatings, as so produced, in addition to being abrasion resistant, are colored in metallic gray, brownish gray and dark brown colors, which are attractive for architectural and other uses of aluminum. Consequently, more practical methods of producing similar coatings are demanded.

It is a general object of this invention to provide, for use in an electrolytic cell for anodizing aluminum, a novel electrolyte and anodizing process or method for producing oxide coatings on aluminum, particularly colored oxide coatings. Another object is to provide such an electrolyte and process suitable for use under a wide range of practical operating conditions. A particular object is to provide such an electrolyte and process for anodizing aluminum which requires no more than moderate cooling of the electrolyte. Another object is to produce abrasion resistant, colored auodic oxide coatings on aluminum at moderate cost. A specific object of the invention is to produce uniform, abrasion-resistant, lightfast, colored anodic oxide coatings on aluminum, with moderate voltage and temperature requirements.

In accordance with the invention, it has been found that an electrolyte consisting essentially of 4-sulforesorcinol, together with sulfate provided by at least one compound selected from the group consisting of sulfuric acid and water soluble sulfates and bisulfates, and water, may be advantageously used in an electrolytic cell for anodizing aluminum.

The 4-sulforesorcinol may be employed in a very small amount, e.g. less than grams per liter, but preferably at least 10 grams per liter or more are employed. The solution limit is in excess of 75 percent by weight, and even a substantial undissolved amount will not prevent anodizing. For production of colored coatings at moderate temperatures, at least 25 grams per liter is desirable, and very satisfactory results have been obtained when 4- 3,143,485 Patented Aug. 4, 1964 ice sulforesorcinol is present in an amount of about 50 to 125 grams per liter. This acid is more expensive than sulfuric acid, so larger amounts would not ordinarily be preferred from a cost standpoint.

The sulfuric acid, or equivalent sulfate, or mixtures thereof, may likewise be employed in a very small amount, e.g. less than 1 gram per liter (calculated as H may be employed, but preferably at least 1 gram per liter or more is employed. Somewhat larger amounts will ordinarily be employed, since the voltage requirements for the same anodizing current density (with consequent elec trical equipment, and electrolyte cooling cost factors to be considered) are reduced in such cases. However, for the production of colored coatings at moderate temperatures, an amount less than 15 grams per liter is desirable, and very satisfactory results have been obtained when sulfuric acid or soluble sulfate is present in an amount of about 2 to 10 grams per liter. The sulfate content may be conveniently provided by sulfuric acid, an alkali metal or ammonium sulfate or bisulfate such as sodium sulfate, sodium bisulfate or ammonium bisulfate, a heavy metal sulfate or bisulfate such as ferrous sulfate, an organic sulfate or bisulfate such as aniline sulfate, or any other water soluble sulfate or bisulfate such as hydrazine sulfate. Double sulfates may also be used.

While various proportions of 4-sulforesorcinol and sulfate may be used, it has been found that the production of uniform, colored coatings at or near room temperature is most readily obtained when the weight ratio (R) of 4-sulforesorcinol to sulfate (calculated as H 80 is at least that expressed by the formula: R=4.5+S/20, Where S is the number of grams per liter of 4-sulforesorcinol in the electrolyte. Electrolytes with lower ratios of these acids, even less than 1:1, are also useful, however. Generally, with the lower ratios, lightly colored or almost colorless coatings may be obtained at or near room temperature. More abrasion resistant, colored coatings may be obtained at lower temperatures which are not, nevertheless, required to be as low as with sulfuric acid electrolytes devoid of the 4-sulforesorcinol.

The following is a specific example of a preferred electrolyte suitable for use in accordance with the invention:

Electrolyte A Component: Grams per liter 4-sulforesorcinol 75 sulfuric acid 6 Water Balance Grams per Liter Component 7 B C D isulforesoreinol 1 5 balance in each case Water.

"Grams per Liter Component 4-sulforesorcinol 35 80 Sulfuric avid 100 50 balance in each case As indicated above, electrolytes for use in accordance with the invention may be employed under a wide variety of operating conditions. A temperature at or near room temperature, e.g. 75 F., may be employed. However, either extremely low temperatures or rather high temperatures may also be employed, i.e. temperatures between about 40 and 110 F. Very satisfactory results in producing colored coatings with preferred electrolyte compositions such as Electrolytes A or D, above mentioned, with a moderate amount of cooling being required, may be obtained with temperatures between about 60 and 90 F.

Current densities may extend over a wide range, for practical purposes as low as 9 amperes per square foot or even less, and as high as 144 amperes per square foot or even more, depending on size, shape and composition of the aluminum article. Preferably, the current density is between 12 and 36 amperes per square foot. The current may be either AC. or DC, or a current of undulating characteristics, but DC. is quite satisfactory.

Time of treatment depends upon current density and thickness of coating desired, as coating thickness is generally a function of anodizing current density and time. Abrasion resistant coatings for outdoor service are often made in a thickness of 0.4 to 2 mils (0.0004 to 0.002 inch). However, coatings may be made in any substantial thickness desired, e.g. 0.1 mil or greater.

Illustrative of particularly suitable operating conditions, Electrolyte A above mentioned has been operated at 75 F., at 24 amperes per square foot, with voltages starting at about 31 volts and generally extending up to about 80 volts over a 30 minute period, depending on the composition of the aluminum being coated, to produce abrasion resistant, colored oxide coatings on a variety of aluminum articles. As examples of the colored coatings produced on various types of aluminum, it is noted that Electrolyte A, operated under the conditions just noted, for time suflicient to produce coatings about 1 mil thick, produced the following results as to color:

The color stability of the colored coatings was very high after exposure in the Fade-O-Meter as compared to the color stability of coatings made at 25'l32 F. in the 4 conventional sulfuric acid electrolyte. Lighter and darker shades of these colors were obtained with thinner and thicker coatings, respectively.

As an example of the abrasion resistance of the coatings, a coating produced on 1100-Hl8 aluminum under the same conditions, with Electrolyte D, exhibited an abrasion resistance value of 484 grams per mil, as obtained by the ASTM D658-44 method of test.

Coatings produced in accordance with the invention may be sealed, or dyed or pigmented and sealed, by conventional procedures. Thus, dyes or pigments may be used to modify the colors produced by the anodizing methods described herein. Etching or brightening treatments may be given the aluminum surfaces prior to anodizing, when desired. The aluminum article treated may, of course, be composed of aluminum alone, an aluminum coated or clad product, a composite aluminum product, or any other form of product presenting an aluminum surface for anodizing.

What is claimed is:

l. A method of anodizing aluminum, comprising making an aluminum article anode in an electrolyte consisting essentially of at least 10 grams per liter of 4-sulforesorcinol,

at least 1 gram per liter and less than 15 grams per liter of sulfate (calculated as H provided by at least one compound selected from the group consisting of sulfuric acid and water soluble sulfates and bisulfates, and

water, the weight ratio (R) of 4-sulforesorcino1 to sulfate (calculated as H 80 being at least that expressed by the formula: R=4.5+S/20, where S is the number of grams per liter of 4-sulforesorcinol in the electrolyte,

for a time sufficient to produce a colored anodic oxide coating at least 0.1 mil thick.

2. A method in accordance with claim 1 in which the electrolyte is maintained at a temperature between about 40 and F., and

the current density is maintained between about 9 and 144 amperes per square foot for a time suilicient to produce a colored anodic oxide coating at least 0.4 mil thick.

3. A method in accordance with claim 2 in which the 4-sulforesorcinol is present in an amount of at least 25 grams per liter, and the sulfate is provided by sulfuric acid.

4. A method of anodizing aluminum, comprising making an aluminum article anode in an electrolyte consisting essentially of about 50 to grams per liter of 4-sulforesorcinol about 2 to 10 grams per liter of sulfate (calculated as H 30 provided by at least one compound selected from the group consisting of sulfuric acid and water soluble sulfates and bisulfates, and

Water, the weight ratio (R) of 4-sulforesorcinol to sulfate (calculated as H 50 being at least that expressed by the formula: R=4.5+S/20, where S is the number of grams per liter of 4-sulforesorcinol in the electrolyte,

while the electrolyte is maintained at a temperature of between about 6'0 and 90 F., and

the current density is maintained between about 12 and 36 amperes per square foot,

for a time of treatment sufficient to produce an anodic oxide coating at least 0.4 mil thick,

whereby a colored anodic oxide coating is produced on'the surface of the aluminum article.

5. A method of anodizing aluminum, comprising making an aluminum article anode in an electrolyte consisting essentially of about 50 to 125 grams per liter of 4-sulforesorcinol,

about 2 to 10 grams per liter of sulfuric acid, and

water,

the weight ratio (R) of 4-sulforesorcinol to sulfuric acid being at least that expressed by the formula: R:4.-5+S/20, where S is the number of grams per liter of 4-sulforesorcinol in the electrolyte,

while the electrolyte is maintained at a temperature of between about 60 to 90 F., and

the curernt density is maintained between about 12 and 36 amperes per square foot,

References Qited in the file of this patent UNITED STATES PATENTS Korpiun Mar. 4, 1941 Deal et a1. Apr. 24, 1962 

1. A METHOD OF ANODIZING ALUMINUM, COMPRISING MAKING AN ALUMINUM ARTICLE ANODE IN AN ELECTROLYTE CONSISTING ESSENTIALLY OF AT LEAST 10 GRAMS PER LITER OF 4-SULFORESORCINOL, AT LEAST 1 GRAM PER LITER AND LESS THAN 15 GRAMS PER LITER OF SULFATE (CALCULATED AS H2SO4) PROVIDED BY AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF SULFURIC ACID AND WATER SOLUBLE SULFATES AND BISULFATES, AND WATER, THE WEIGHT RATION (R) OF 4-SULFORESORCINOL TO SULFATE (CALCULATED AT H2SO4) BIENG AT LEAST THAT EXPRESSED BY THE FORMULA: R=4.5+S/20, WHERE S IS THE NUMBER OF GRAMS PER LITER OF 4-SULFORESORCINOL IN THE ELECTROLYTE, FOR A TIME SUFFICIENT TO PRODUCE A COLORED ANODIC OXIDE CATING AT LEAST 0.1 MIL THICK. 